Memory cells often have a finite erase/write capability. The process of wear leveling is designed to reduce the likelihood of premature failure in memory arrays by preventing any one region of memory being used much more frequently than any other region.
In addition, memory cells, even those considered as volatile, have been shown to have some non-volatile ‘remanence’ characteristics. Remanence characteristics result in having some residual information from previously stored values even after a power off cycle. By probing memory cells, it is therefore possible to partially recover bits of the secret keys that were stored for a long time but have gone through a power off cycle since. A security breach may occur by sophisticated direct probing of memory cells. If an attacker is aware of the physical location of the key or any other relevant data being sought, the attack becomes easier.